Efl.Class (in practice, the return from the *_class_get() functions) can
be used as argument to functions, like in Efl.Object.provider_find and
Efl.Ui.Widget_Factory.item_class(get/set).
This commits adds support by representing Efl.Class instances
as System.Type in the C# API, allowing someone to do things like:
`factory.ItemClass == typeof(MyFramework.MyButton)`
It also supports user-defined classes that inherit from efl classes.
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